Thermoplasma acidophilum is a thermophilic and acidophilic mycoplasma, and is unique among known prokaryotes in that it has a histone-like protein associated with its DNA. The protein is similar to eukaryotic histones, but does not appear to be closely homologous with any known histone class. One unique feature is the high content (20%) of aspartamide and glutamide. Several possible functions of the histone-like protein will be tested: (1) Does the protein inhibit depurination of the DNA? Depurination should be a problem in the intracellular conditions of high temperature and acidity (pH 5.6). (2) Does the histone significantly stabilize the DNA against thermal denaturation? (3) Does the histone protect the DNA from aggregation with cytoplasmic proteins? This problem is potentially severe, since the cytoplasmic proteins appear to be basic in nature, and the intracellular ionic strength is unusually low. (4) Does the histone condense the DNA? (5) Does the histone affect the template capacity of the DNA? (6) Does the histone contribute to the general intracellular ionic strength? (7) And finally, what is the role of the high amide content? Is this a modification that controls how tightly the histone binds to the DNA? These studies may contribute to an understanding of the evolutionary origins both of histones and of eukaryotic cells. The findings may also be related to the functions of eukaryotic hormones.